Locking is a mechanism used by Oracle to ensure data integrity and consistency in a multi-user environment. It prevents simultaneous access to data that could lead to conflicts or corruption. Oracle employs various types of locks, each with a specific purpose and behavior.
Types of Locks in Oracle Database
1. Automatic Locks:
• DML Locks (Data Manipulation Language): Automatically acquired when performing DML operations (INSERT, UPDATE, DELETE). These locks ensure that only one transaction can modify a specific row at a time.
• Row-Level Locks (TX): Applied to rows being modified. They prevent other transactions from modifying the same rows until the transaction is complete.
• Table-Level Locks (TM): Acquired when a DML statement is executed. These prevent other transactions from performing conflicting operations on the same table.
• DDL Locks (Data Definition Language): Acquired automatically during DDL operations (CREATE, ALTER, DROP). These locks ensure that the structure of database objects is not altered while they are being used.
• Exclusive DDL Locks: Prevents other transactions from accessing the object being modified until the DDL operation is complete.
• Shared DDL Locks: Allow multiple transactions to read the metadata of an object concurrently, but prevent modifications.
2. Manual Data Locks:
• Explicit Locks: Applied manually by users to enforce specific locking policies.
• SELECT … FOR UPDATE: Locks the selected rows for update. This prevents other transactions from modifying or deleting the locked rows until the lock is released.
• LOCK TABLE … IN [EXCLUSIVE|SHARE] MODE: Manually locks a table to control concurrent access. Exclusive mode locks prevent any other operations on the table, while share mode locks allow concurrent reads but prevent modifications.
3. User-Defined Locks:
• DBMS_LOCK Package: Allows users to create and manage custom locks that do not directly correspond to rows or tables but can be used to control access to application-defined resources.
• DBMS_LOCK.ALLOCATE_UNIQUE: Creates a unique lock identifier.
• DBMS_LOCK.REQUEST: Requests a lock for the given identifier.
• DBMS_LOCK.RELEASE: Releases the specified lock.
Understanding Lock Modes
Oracle uses various lock modes to control access to resources:
• Exclusive Lock (X): Allows only the locking transaction to access the locked resource. No other transaction can access it until the lock is released.
• Share Lock (S): Allows multiple transactions to read the locked resource but prevents modifications.
• Row-Exclusive Lock (RX): Allows concurrent access for reads and writes to different rows in the table but prevents other transactions from acquiring a full table lock.
• Row-Share Lock (RS): Also known as a sub-share lock, it allows concurrent access for reads and writes but does not allow exclusive locks.
Lock Conflict Resolution
When multiple transactions attempt to access the same data, Oracle uses a sophisticated mechanism to resolve conflicts:
• Deadlock Detection: Oracle automatically detects and resolves deadlocks by rolling back one of the conflicting transactions.
• Waits and Timeouts: Transactions wait for a specified period for the required locks to be released. If the wait time exceeds a certain threshold, the transaction is rolled back.
Best Practices for Locking
• Minimize Lock Duration: Keep transactions short to reduce the time locks are held.
• Avoid Unnecessary Locks: Use locking mechanisms only when necessary to prevent conflicts.
• Handle Deadlocks Gracefully: Design applications to handle deadlocks and implement retry logic if necessary.
By understanding and effectively using Oracle’s locking mechanisms, database administrators can ensure data consistency and integrity while maintaining optimal performance in a multi-user environment.
